Mechanism of nonresonant energy transfer between ions inSrF2:Er3+

The temperature and magnetic field dependence of the nonresonant energy transfer rate is measured for two ions that exist as a well-defined dimer in ${\mathrm{SrF}}_2$:${\mathrm{Er}}^{3+}$. The two ions are crystallographically inequivalent, so the energy transfer can be observed directly and unique transfer rates are obtained without modeling the radial dependence of energy transfer. The transfer is shown to result from a single-phonon-assisted process at low temperatures and a two-phonon-assisted resonant or Orbach process at higher temperatures. No evidence is found for nonresonant two-phonon processes.